1. Field Of The Invention
The present invention relates to an improved sensor device for detecting liquid, preferably in drop form, on a glass screen such as a windshield.
2. Description Of The Related Art
Sensor devices of this type are particularly intended to register the amount of liquid present on the front or rear windshield of a motor vehicle in a directly related form and within a defined unit of time and then, depending on the degree of wetness, to automatically influence an associated windshield wiper system.
There has been previously described a control device for a windshield wiper motor, in which a light conducting element is attached to a windshield by means of light conducting adhesive and provided with a beam emitter and receiver. The beams are emitted and received via two lenses. The beam emitter and receiver are so arranged that any beam emitted from the output element of the emitter is at an angle of approximately 90.degree. relative to the beam entering the input element of the receiver defined at the point where the beam enters or leaves the light conducting element.
In this version, in order to achieve multiple reflection of the beams, a reflector means is situated in an area which is between the two lenses, consisting of a part of the surface of the windshield which faces the light conducting element and an air space next to it. Here, however, the difficulty arises that the sensing area is small, and condensation settles on the reflector surface of the windshield out of the air space, because of temperature differences between the inside and outside of the windshield. This condensation then changes the reflective characteristics of the windshield, and the intensity of the beam entering the receiver is not a true indicator of the actual amount of rain or other liquid drops present on the windshield surface. It follows that the signal which is emitted from the receiver is not appropriate and than the windshield wiper system is not activated as intended.
A further sensor device variant is known in which the reflector means consists of a metal plate running through the light conducting element parallel to the windshield. With this design, however, haircracks can appear in the material of the light conducting element for the following reasons. The expansion coefficients of this material--glass, PLEXIGLASS or similar--and that of the metal plate reflector means, which is quite thick (approximately 1 mm) and preferably made of aluminum, are different, and the temperature to which a windshield-mounted sensor is exposed can vary between -40.degree. to +90.degree. C. Any haircracks can have a considerable effect on the path of the beams and also admits moisture, possibly in the form of dirty water. Total failure of the device can be the result.
During manufacturing, the cooling rates within the glass or plexiglass can also vary depending on the material's proximity to the cold metal plate reflector means in the tool, leading to tensions and again, later cracking or splitting between the plate and the main material, with resultant beam and signal distortion. These problems can be reduced if the reflector plate is made thinner, but this is only possible to a limited extent, as the plate must be capable of being precisely held in the tool and must not deform when the material of the light conducting element is applied.